The present invention relates to the control of scaling deposits, such as alkaline earth metal sulfates, on a surface and compositions which have the capability of controlling alkaline earth metal sulfates on surfaces, and in porous media. The present invention also relates to the drilling industry and more particularly relates to the completion fluids used in the completion of a well for the recovery of hydrocarbons or other materials.
Alkaline earth metal sulfates, such as barium sulfate (also known as barite) are scales frequently seen in oil field operations, most often occurring due to temperature drops occurring in production or in the mixing of incompatible waters from sea water injection. Barium sulfate, unlike carbonate scales, tends to form quickly once saturation conditions are reached and does not readily dissolve in currently available solvents.
The alkaline earth metal sulfates generally form as scales on the surfaces of well bores, perforation tunnels, near well bore regions, the tubing in a well, and other surfaces and eventually can build up to a point where the amount of hydrocarbons being produced from the well is significantly reduced. In the past, a well would have to be shut down in order for the barium sulfate and other alkaline earth metal sulfates to be removed by mechanical and/or chemical treatments. For instance, a mechanical scrubber could be inserted into the well to remove the scaling or chemicals, such as solvents, could be used to remove the scaling. When barium sulfate occurs within a formation, there is no current truly effective treatment for removal. Stimulation of the well is attained by dissolving the rock around the barite. The current technology available for controlling alkaline earth metal sulfate buildup in wells and other surfaces has thus far proved not to be economical due to slow dissolution rates resulting in extended shut in and the need to shut the well in and/or the inability of the current technology to adequately remove a sufficient degree of the alkaline earth metal buildup.
In drilling operations, such as the drilling that occurs in oil field operations, drilling fluids are designed/formulated to serve several functions. These functions include acting as a lubricant to the drill bit to reduce wear and friction during drilling and also to seal the formation surface by forming a filter cake. Currently, in the industry, both oil-based muds (OBMs) and water based muds (WBMs) are typically used. More commonly, synthetic based muds (SBMs) are also used in drilling operations. In the drilling fluid, agents for lubrication will be present as well as weighting materials in order to achieve a density that is typically greater than the surrounding pressure in the well bore. Furthermore, the drilling fluid will also contain a sealing or fluid loss agent, such as calcium carbonate and polymers, in order to form the filter cake on the formation surface of the well bore. In addition, when the drilling fluids are used during drilling, the drilling fluid will also contain drilling fines, such as shale and sandstone fines. During the drilling operations and afterwards, the filter cake seals the formation surface of the well bore so that the well bore can be completely formed without any leakage from the formation surface into the well bore and/or without any leakage of the drilling fluids into the formation surface. While the filter cake is beneficial for these reasons, once the drilling is completed, and the recovery of hydrocarbons is the next step, the filter cake can act as a severe impediment to the recovery of hydrocarbons. For instance, the filter cake can prevent the recovery of hydrocarbons from the formation surfaces which have been blocked or sealed by the filter cake. Furthermore, when injectors are used to retain reservoir pressures, the injection of sea water, for instance, can be significantly reduced due to the filter cake preventing the sea water from entering the formation and hence enhancing recovery of hydrocarbons. This invention preferably refers to wells that are drilled to either recover hydrocarbons or injector wells used for pressure retention in a reservoir which will either use sea water, produce water, or waste water injection such as in disposal wells. In all these cases, it is useful to retain optimum injection or production rates by first removing all remnants of the filter cake first used to drill the well. Accordingly, the industry prefers to remove the filter cake from the well bore in order to optimize productivity. If the filter cake is not removed, the filter cake can block the pores that are part of the formation surface of the well bore which will interfere with the recovery of hydrocarbons. The removal of the filter cake can be even a more difficult problem when the drilling fluid contains barite as the weighting material. Typically, in many drilling operations, the drilling fluid can contain up to 50% by weight of a weighting material such as barite. Barite, also known as barium sulfate, and other alkaline earth metal sulfates are not easily dissolved and are not easily suspended in liquids. Thus, the removal of barite or other alkaline earth metal sulfates that may be present in drilling fluids can be a significant problem. In many drilling operations, the drilling fluid can contain up to 5% by weight of a pore bridging material such as calcium carbonate. Calcium carbonate (CaCO3) is typically a blend of particle sizes with a particle size distribution designed to optimize the bridging of the pores found in the formation. The pore size distribution of the formation is determined from its permeability, preferably by direct porosity and permeability measurements of core plugs extracted from the reservoir.
When the drilling fluid is an oil-based mud, and one that contains pore bridging materials such as calcium carbonate, the ability to displace the oil-based mud and dissolve the filter cake with a completion fluid can be difficult. Typically, the oil-based components of the filter cake must be displaced in order to allow water based completion fluids to contact the surface of the filter cake which typically can contain calcium carbonate. Calcium carbonate, for instance, is best removed by dissolution at a low pH and thus any completion fluid used must have certain properties to be totally effective in removing the filter cake. In other words, an optimum completion fluid is needed which can operate at a low pH, and yet have sufficient density. More preferably, an aqueous-based completion fluid is desired so as to limit any damage to the formation, and more preferably a solids-free material with the minimum of additives would be more preferred in order to avoid any fouling of the well bore.
Once drilling operations have been completed, the well is prepared for the completion operations whereby the mud used for drilling is often displaced by a completion fluid. Completion fluids are typically water based clear fluids and are formulated to the same density as the mud used to drill the well in order to retain the hydraulic pressure on the well bore. There are numerous methods of completing a well, amongst which are open hole completions, pre-drilled, liner, and gravel packed screened systems. The clear fluids are typically halide based brines such as calcium bromide, calcium chloride, and zinc bromide; or organic based brines such as the formate based fluids. While the completion fluid displaces the drilling fluid, the filter cake is still in place and accordingly, there is a need to provide better technology to overcome the above-described problems and preferably to provide technology which can successfully remove the filter cake from the formation surface of the well bore, including any CaCO3, polymer fluid loss agents and/or alkaline earth metal sulfates that may be present as part of the drilling fluid and at the same time retain density in the well bore.
Also, there is a need to provide better technology to overcome the various problems described above, and preferably to provide technology which can avoid the need for a well shut in and so remove scaling deposits, such as alkaline earth metal sulfate buildup.